In the first movie Tyrell tells Deckard that Rachel is a new experimental model that doesn’t know that she is a replicant. And Tyrell is VERY proud of this. A FIRST!! In the second movie they talked about Rachel as unique because she gave birth. That’s the center theme of the second movie. Basically she was a one-off model.

If she is that (a one off-model), then Deckard, being old, and capable of producing a baby too (had to have semen to get her pregnant) would mean she was not as unique as Tyrell said, as Deckard was manufactured before Rachel.

If Deckard was a replicant, then the premise of both movies falls apart: 1) Rachel was not unique, and 2) the cross link of two species, creating a baby.